Solenoid valves are commonly used in medical apparatus such as massage devices for patients. U.S. Pat. No. 5,992,461 discloses a typical solenoid valve which comprises a valve body with two ports and a solenoid. The solenoid comprises a flux return frame, a bobbin with an inner hole, a stator installed in one end of the hole of the bobbin and forming another port, a winding wound on the bobbin, and a plunger installed in the other end of the hole of the bobbin. The plunger defines an inner space within which a pair of plugs and an internal spring are installed. The internal spring is compressed between the plugs for holding the plugs in position. An external spring is arranged between the plunger and the stator. The frame has two ends, one fixed to the stator and the other fixed to the valve body. In the energized state, the plunger is attracted to the stator and one of the plugs inside the plunger closes the port of the stator and the two ports of the valve body are in communication with each other. In the normal state, i.e. when the solenoid is deenergised, the plunger is pushed away from the stator by the external spring and the other plug inside the plunger closes the first port of the valve body. A channel is formed between an outer surface of the plunger and an inner surface of the valve body for communicating the port of the stator with the second port of the valve body in the normal state. A big gap is therefore formed between the outer surface of the plunger and the edge of the end of the frame. The big gap has a large magnetic resistance, and reduces the attraction force generated between the stator and the plunger when the solenoid is energized. In order to meet the required attraction force, the winding needs high power consuming a relatively large amount of electrical power and generating heat that needs to be dissipated.
Hence, there a desire for an improved solenoid valve which can generate sufficient attraction force with less power.